1. Field of the Invention
The invention relates generally to signal processing apparatuses and methods. More particularly, it relates to a low-delay signal processing system and method which are based on highly oversampled digital processing so as to achieve low throughput delay.
2. Description of the Prior Art
As is generally known to those skilled in the art, audio filtering with low delay has been performed traditionally in an analog domain (i.e., analog signal processing). However, analog signal processing (audio filtering) suffers from a number of drawbacks. For example, one problem is the difficulty of implementing analog filter components with high precision or accuracy. In other words, analog filtering requires the use of components that generally cannot be closely matched, especially in integrated analog filters. Further, analog filter circuits are susceptible to signal drift and to environmental noise build-up, especially when a high-order analog filter system is implemented.
In addition, analog filter circuits can consume a large amount of power when the noise must be kept low. Also, these circuits have the disadvantage of being difficult in maintaining high linearity. Nevertheless, analog signal processing is still often used in applications where filtering is required with a minimum delay. Such filters are used in real-time control systems for active noise cancellation or feedback cancellation applications.
For example, one application for low-delay signal processing where active feedback cancellation is used is in a hearing aid system 10, as illustrated in FIG. 1A. The hearing aid system 10 includes a speaker 12 disposed into the ear 14 and a microphone 16 arranged on the outside of the hearing aid 18. There exists an acoustic path along line 20 having leakage around the hearing aid 18 from the speaker 12 back to the microphone 16. This leakage causes the hearing aid to have feedback which creates a howling or “whistling” sound. This “whistling” when it occurs can be eliminated by canceling or subtracting out electronically with a model of the feedback path. This feedback path is desired to be very fast. Thus, the present invention would be useful in providing a feedback path with low delay so as to avoid any howling.
FIG. 1B shows a simplified block diagram of the hearing aid system 10 of FIG. 1A, which utilizes the present invention. The hearing aid system 10 includes signal processing feedback cancellation block 22, which is designed to model the transfer function of the feedback path in order to cancel out the acoustic leakage inherent in the system. A summer 24 subtracts a feedback path model signal 26, which represents the acoustic leakage of the system 10, from the audio input on line 28 coming from the microphone 16. The difference signal 30 is then processed by the audio signal processor block 32 and generates an audio output signal 34 to the speaker 12. The audio output signal 34 is also fed back to the feedback cancellation block 22.
FIG. 2 is a more detailed block diagram of the hearing aid system 10 of FIG. 1B. The hearing aid system 10 includes a delta-sigma modulator 36 having its input connected to receive the analog audio signal from the microphone 16 and having its output connected to a first input of the summer 24. The output of the summer 24 is fed to a decimation filter 38 having an output that is connected to a digital signal processor block 39. The output of the digital signal processor block 39 is fed to an interpolation filter 40 which has an output that is connected to a delta-sigma modulator 42. The output of the modulator 42 is connected to the speaker 12.
The signal processing feedback cancellation block 22 includes an oversampled model block 44 of the hearing aid and an adjustable delay block 46. The output of the interpolation filter 40 is connected to the input of the oversampled model block 44. The output of the block 44 is fed to the second input of the summer 24 via the adjustable delay block 46.
Another application of where active noise cancellation is used is in a headphone system 48, as depicted in FIG. 3. The headphone system 48 includes an earpiece 50 (only one being shown) which has a speaker 52 aimed at an ear 54 and a microphone 56 arranged on the outside of the earpiece. There exists an acoustic path along line 58 having leakage around the earpiece 50. The headphone system uses noise cancellation of the feedforward type so as to reduce the noise heard by the user wearing the headphone. This noise cancellation is achieved through modeling the feedforward path by placing a microphone on the outside of an earpiece. A compensating signal, which is the same amplitude as the leakage but opposite in phase, is then inserted so as to cancel out the noise.
FIG. 4 is a block diagram of the feedforward headphone system 48 of FIG. 3. The headphone system 48 includes a delta-sigma modulator 60 having its input connected to receive the analog input signal from the microphone 56 and having its output connected to a decimation filter 62. The output of the decimation filter 62 if fed to a record operation block 64. At the same time, a play operation block 66 has its output connected to an interpolation filter 68 which has an output that is connected to a delta-sigma modulator 72 having an output that is connected to the speaker 52. An analog filter 61 of the headphone system is also connected to the output of the microphone 56. The output of the analog filter 61 is also fed to the speaker 52. The analog filter 61 cannot be connected after the decimation filter 62 and before the interpolation filter 68 due to the delay that would be encountered. As a result, the analog filter 61 is interconnected before the input of the decimation filter 62 and after the output of the interpolation filter 68. The analog filter 61 is formed of Resistor-Capacitor (RC) elements and its adjustments thereof consume a large amount of power. Optionally, the delta-sigma modulator 60, decimation filter 62 and record operation block 64 shown within the dotted line 65 would be omitted for the case of a play-only system.
In FIG. 4A, there is shown a prior art analog filter 71 for use in noise cancellation applications. The analog filter 71 is formed of a plurality of series-connected biquad filter stages 74a-74c. The biquad filter stages suffer from the disadvantage of consuming a large amount of power. Further, the biquad filter stages are difficult to accurately set and require the use of large capacitors.
In FIG. 4B, there is shown a block diagram of a prior art digital processing system 76 for use in noise cancellation applications. The digital processing system includes a delta-sigma modulator 78 having its output connected to a decimation filter 80. The output of the decimation filter is connected to a plurality of series-connected biquad filter stages 82a-82c. The output of the filter stage 82c is fed to an interpolation filter 84 having an output that is connected to a delta-sigma modulator 86. Since all of the biquad filter stages 82a-82c are being operated at a low sampling rate, they are relatively easy to set accurately and use a small amount of power. However, they suffer from the drawback of having large delays due to the inherent delays in the decimation and interpolation filters. As a result, this prior art digital processing system is generally not very useful in many real-time applications where a low delay is desired.
Still another application of where noise cancellation (feedback) is used is in a headphone system 510, as shown in FIG. 5. The feedback headphone system 510 is substantially identical to the headphone system 48 of FIG. 3, except the microphone 576 is located on the inside of the earpiece 578. Noise cancellation (feedback) is provided so as to cancel the leakage signal from the speaker 580 to the microphone 576. In this case, it is even more critical that the delay be minimized so as to avoid causing system instability, thereby producing oscillations.
Still yet another application of where noise cancellation is used is in an active control system 610 of acoustic energy from a noise-producing machinery, as depicted in FIG. 6. Noise cancellation is utilized to cancel the rumbling noise coming from the speaker 682 in the air duct system 684. Again, in order to provide loop stability, there must not be any delays. A compensating signal from the speaker 682 but opposite in phase to the noise coming down the air duct is used to produce noise cancellation. The noise is inputted to the microphone 686.
Yet still another application of where noise cancellation is used is in a motor control system 710, as shown in FIG. 7. The motor control system 710 includes a power amplifier 712 and a motor 714. In the feedback path, there are provided a mechanical system 716 for a position motor, a position sensor 718, and a compensating on filter 720 (analog). A summer 722 subtracts the compensating signal 724 from the input signal 726 and generates an error signal 728 for driving the power amplifier 712 in order to control the motor 714.
However, modern control systems have now been implemented with digital filters rather than analog filters. There are inherent delays that are associated with digital type filters. Therefore, there is a need to provide signal processing in the digital domain, but yet realizes low delays for use in noise cancellation applications of the feedforward or feedback type. Unfortunately, conventional digital signal processing has been found to be inappropriate for real-time applications due to the fact that they are based on relatively slow sequential signal processing.
Recognizing the drawbacks of the analog signal processing discussed above and the unacceptable use of conventional digital signal processing for real-time applications, it would be desirable to provide a low-delay signal processing system which is based on highly oversampled digital processing.